1. Field of the Invention
The present invention pertains to an image-forming apparatus, and a method thereof, such as a copier or printer using the electrophotographic method. More particularly, an image-forming apparatus that has an intermediate transfer belt, wherein a toner image formed on a photoreceptor, via a electrophotographic process, is transferred to the intermediate transfer belt in a first transfer area, and when first transfers of prescribed toner images are completed, the toner images obtained in the first transfers undergo a second transfer to a recording medium in a second transfer area.
2. Description of Related Prior Art
Some electrophotographic image-forming apparatuses carry out a first transfer of the toner images formed on the photoreceptor to an intermediate transfer unit in a first transfer area, and when first transfers of prescribed toner images to the intermediate transfer unit are completed, they perform a second transfer of the toner images obtained in the first transfers from the intermediate transfer unit to a recording medium in a second transfer area.
An image-forming apparatus that forms multi-color images (full color images, for example) would be a representative example of the image-forming apparatus described above. In a multi-color image-forming apparatus, in order to overlap the toner images of each color that are sequentially formed on the photoreceptor before they are finally transferred to a recording medium, they are sequentially overlapped and transferred to an intermediate transfer unit in a first transfer area, so that the overlapping toner images thus formed on the intermediate transfer unit may be transferred to the recording medium in a second transfer area and fused.
In a full-color image-forming apparatus, for example, toner images of cyan, yellow, magenta and black are sequentially formed on the photoreceptor, such as a photoreceptor drum, via the electrophotographic process. Each time a toner image of a color is formed, it is first transferred to an intermediate transfer unit. After toner images of each color have been transferred to the intermediate transfer unit and overlapping toner images have been formed on the intermediate transfer unit, these overlapping toner images then undergo a second transfer to a recording medium and are fused.
An intermediate transfer belt that is suspended over a group of rollers, including a belt drive roller and first transfer roller, would be one example of the intermediate transfer unit described above.
Some of the issues to be resolved by the present invention are discussed below. However, a conventional image-forming apparatus adopting such an intermediate transfer belt has the following problem.
The intermediate transfer belt is suspended over a group of rollers including a belt drive roller, that are driven to rotate, and a first transfer roller that puts the intermediate transfer belt into contact with the photoreceptor surface in order to carry out the first transfer. However, when the belt stops, the intermediate transfer belt that is suspended over a group of rollers in this way may be subject to creep buckling at the area that is suspended over a small-diameter roller. If a toner image is transferred to this creep-buckled area in a first transfer, a defective first transfer can easily occur due to this deformation or to the fluctuation in the electrical resistance caused by the deformation. Thereby, causing the image finally obtained on the recording medium to also turn out defective.
To resolve these problems, while it is possible to increase the diameters of the rollers over which the belt is placed, such a solution would not satisfy the current demand for size reduction and compactness in an image-forming apparatus.
In order to achieve size reduction and compactness and to obtain nipping useful for the first transfer from the photoreceptor to the intermediate transfer belt, a small-diameter backup roller that presses the belt against the photoreceptor from the inside is sometimes placed near to and upstream from the first transfer roller in terms of the direction of movement of the belt. However, when this construction is used, the part of the belt between the small-diameter backup roller and the first transfer roller--particularly the part which is suspended over the small-diameter backup roller--often becomes susceptible to creep buckling.